1. Field of the Invention
The present invention relates to pigmented dispersions and inkjet inks comprising colour pigments that are stabilized by polymeric dispersants in a non-aqueous medium using specific dispersion synergists.
2. Description of the Related Art
A dispersant is a substance for promoting the formation and stabilization of a dispersion of pigment particles in a dispersion medium and for substantially reducing the dispersing energy required. Dispersed pigment particles may have a tendency to re-agglomerate after the dispersing operation, due to mutual attraction forces. The use of dispersants counteracts this re-agglomeration tendency of the pigment particles.
The dispersant has to meet particularly high requirements when used for inkjet inks. Inadequate dispersing manifests itself as increased viscosity in liquid systems, loss of brilliance and/or hue shifts. Moreover, particularly good dispersion quality is required to ensure unimpeded passage of pigment particles through the nozzles of a print head, which are usually only a few micrometers in diameter. In addition, pigment particle agglomeration and the associated blockage of the printer nozzles has to be avoided in the standby periods of the printer.
Polymeric dispersants typically contain in one part of the molecule so-called anchor groups, which adsorb onto the pigments to be dispersed. In a spatially separate part of the molecule, polymeric dispersants have polymer chains compatible with the dispersion medium, thus stabilizing the pigment particles in the dispersion medium. Typical polymeric dispersants include graft copolymer and block copolymer dispersants.
In aqueous inkjet inks, the polymeric dispersants generally contain hydrophobic anchor groups exhibiting a high affinity for the pigment surface and hydrophilic polymer chains for stabilizing the pigments in the aqueous dispersion medium.
The preparation of good thermally stable dispersions with submicron particles is more difficult for non-aqueous pigment dispersions, such as solvent based, oil based and radiation curable inkjet inks. The pigments are especially difficult to disperse when they have a non-polar surface.
These problems have led to the design of very specific polymeric dispersants wherein the anchor groups are pigment derivatives. For example, WO 2007/006635 (AGFA GRAPHICS) discloses pigment dispersions comprising a colour pigment and a polymeric dispersant having via a linking group covalently linked to its polymeric backbone at least one pending chromophore group which has a molecular weight smaller than 90% of the molecular weight of the colour pigment. A disadvantage is that the pending chromophore group must bear some similarity to the chemical formula of the pigment in order to obtain effective dispersion, which as a consequence results in different polymeric dispersants for the different pigments used in an ink set containing yellow, magenta and cyan inks.
Another approach for dispersing pigments with non-polar surfaces in non-aqueous dispersion media is changing the surface to a more polar surface by addition of compounds known as dispersion synergists. A dispersion synergist is a compound that promotes the adsorption of the polymeric dispersant on the surface of the pigment. To be efficient, the synergist should bear some similarity to the pigment. Examples of these dispersion synergists are given in, for example, WO 2007/060254 (AGFA GRAPHICS), EP 1790697 A (AGFA GRAPHICS) and EP 1790698 A (AGFA GRAPHICS)).
However, both approaches lead to a considerable higher cost in composing non-aqueous pigment dispersions for inkjet ink sets, because each pigment dispersion requires anchor groups on the polymeric dispersant or dispersion synergists which chemically resemble the pigments.
For consistent image quality, inkjet inks require a dispersion stability capable of dealing with high temperatures (above 60° C.) during transport of the ink to a customer, jetting at elevated temperatures and changes in the dispersion medium of the inkjet ink during use, for example, evaporation of solvent and increasing concentrations of humectants, penetrants and other additives.
Therefore, it is highly desirable to manufacture low cost non-aqueous pigment dispersions, especially pigmented inkjet inks, exhibiting a high dispersion quality and stability using the same polymeric dispersant in all inks of an ink set.